Photonic bandgap (PBG) structures, which represent the optical equivalent of the energy gap in semiconductors, promise a wealth of new, very compact, optical devices. PBG structures can confine light in very tight regions, allowing for the radiation to be guided or bent around sharp corners with virtually no energy loss. PBGs are expected to reduce the size of optical devices by orders of magnitude and allow for a larger scale of integration.
Indeed, the concept of a PBG is analogous to the periodicity of the atoms or molecules of an ordinary crystal. The PBG comprises an artificial structure in which elements with different refractive indices are disposed in a periodic arrangement. Thus, for a certain range of wavelengths, there are no states for the photons to occupy in the structure. Photons with these wavelengths are thus defined as “forbidden” and cannot propagate. As opposed to opaque materials (such as metals), PBG structures are usually formed of a dielectric, so that no energy is absorbed by the structure.
The presence of a defect in a PBG structure generally results in a “localized state”, that is, a tightly confined region of light energy that must stay within the defect, since it cannot propagate in the structure, and its energy cannot be absorbed by the structure. Therefore, if the defects are appropriately designed and arranged, they can be used to create waveguides with very small dimensions and excellent directional control and light confinement properties. Indeed, many optical devices—such as optical add/drop filters, multiplexers/demultiplexers, resonators, cavities, etc.—have now been realized using PBG structures.
With the emergence of silicon-on-insulator (SOI) technology for the fabrication of optical devices, two-dimensional PBG structures have been developed that may be realized by etching the desired hole pattern in the thin silicon surface layer (generally referred to as the “SOI layer”) so as to form the optical band structure. The photons with appropriate energy will pass through regions of high refractive index (e.g., silicon, or a polysilicon layer disposed over the silicon) interspersed with regions of low refractive index (e.g., the “hole” structures, where the holes are generally filled with commonly-used low index dielectric materials such as oxides or nitrides). To a photon, this contrast in refractive index looks just like the periodic potential that an electron experiences traveling through a silicon crystal. The large contrast in refractive index, as mentioned above, allows for the light to be confined in a very small region.
The diameter and periodicity of the etched holes (subsequently filled with low dielectric material), together with the contrast in effective refractive index between the high index and low index regions, are the parameters that can be modified to produce the desired two-dimensional PBG structure. Indeed, to form a photonic bandgap, the etched holes need to be separated by a distance roughly equal to the photon wavelength divided by the refractive index. The width of the bandgap depends on the contrast in effective refractive index between the two materials in the lattice—with a larger contrast yielding a wider bandgap.
As mentioned above, extremely tight confinement within the waveguide region of a PBG structure makes it possible to bend the light around sharp corners with low energy loss, enabling the formation of very small optical circuits. For example, state-of-the-art silicon-based strip or rib single mode waveguides can be achieved with dimensions on the order of 0.34 μm. However, a significant portion of the energy will reside in the “tail” outside of the core and into the cladding. By virtue of using a PBG structure, little if any energy will be outside of the waveguide boundaries.
Active electro-optic devices including PBGs are currently being fabricated by filling the holes in the structure with a polymer or liquid crystal, then using an applied external signal to re-arrange the periodicity of the material to affect the change in wavelength. Micro-electromechanical systems (MEMS) are also being explored to provide the desired active control. However, both of these arrangements exhibit a relatively slow speed of operation (not acceptable for multiple Gb/s applications), and do not readily lend themselves to high volume manufacture. Other arrangements, to date, require the formation of a resonant cavity within the guiding structure and are therefore extremely wavelength-selective.
Thus, a need remains in the art for a tunable PBG structure that exhibits the requisite speed and manufacturability demands for future, high speed opto-electronic applications.